Growth and Reproduction

Growth

Growth in diatoms occurs unidirectionally and results in a mean
size reduction at each division. The unidirectional growth is a result
of the siliceous nature of the frustule, for it cannot expand directly
within the frustule and therefore must expand along the girdle. There
is a reduction in mean size because each new valve is produced within
the frustule of the parent.

Here is the general mechanism for growth in diatoms:

(1) The valves (the epitheca and hypotheca) start shifting apart
due to the construction of new valve elements back to back within
the parent frustule. In order to allow this movement, the hypocingulum
is constantly being added to and is not fully constructed until just
before the cell actually divides.

(2) When the cell has fully completed mitosis (and the elements
for the new valves are now enclosed within the parent frustule),
the cell can divide. The hypovalve/hypocingulum of the parent frustule
now becomes the epivalve/epicingulum of the daughter cell, and the
parent epivalve/epicingulum becomes the epivalve/epicingulum of the
other daughter cell.

Sexual Reproduction

Due to the mean size reduction at every cell division, the diatom
cell needs to have a way in which to regain its size when it gets
too small. This is done by the formation of an auxospore through
sexual reproduction. The vegetatively reproducing cells can differentiate
into spermatocytes and oocytes. Sexual reproduction can occur in
the form of either oogamy (usually found in centric diatoms) or isogamy
(usually seen in pennate diatoms). The auxospore is a cell that develops
and grows, eventually proceeding to produce a new frustule. Auxospore
formation allows the diatom to restore its size, and vegetative growth
can continue once again.